The idea for the map is based off of those old Fighting Fantasy books where you used dice and a pencil. It gets its name from the first book, The Warlock of Firetop Mountain. Back in the mid to late 90s I sketched out the map as described in the book and decided to make a Doom level based on it. The floor plan I sketched is long lost, so apart from the beginning of inside the mountain, the rest has been newly created from scratch.

A big, BIG, shout out and mad props to Rex Claussen whose advice and help kept me motivated to improve the project. Also, Rex did the preliminary work on sloping the outside of the mountain which really inspired me to take it further, and he provided the rock texture as well... what a top bloke!

Modified the my Firetop Mountain, which was about 10% of what you see here when I first started this project in March 1997. Second modification on the original FTOPMTN.WAD which I released in May 2012.

Build time:

100 hours or so over 5-6 months. I'm slow and my mapping sessions aren't very long, but frequent ;)

I am going to label this as a must play! You can just tell the amount of effort put into this wad by just playing it for 30 seconds, I am surprised this could even be made in doom! this has to be the most non linear map I have ever played, it even has little touches like imp filled forests, stop reading this and play this masterpiece, 5/5

It took me about forty minutes to get through on UV, and the final battle was more of a shock than anything, considering I thought the Cyberdemon was the final fight... I was SADLY... mistaken... The bottom line is, this is an amazing WAD, and the non-linearity to it really makes it shine among one of the best so far this year! 5/5 - K12machinima

You said: "based off of those old Fighting Fantasy books where you used dice and a pencil", you mean "non-linear map." You could have just said: "non-linear map."
Still it is a decent looking map with plenty of good gameplay, and the atmosphere is well done, so 4/5